Graphical Tropical Weather Outlook

Contents

4. Probabilistic Genesis Forecasts

As part of the experimental process, the NHC tested in-house probabilistic TC genesis forecasts during the 2007 and 2008 hurricane
seasons. The purpose of this exercise was to determine if there was sufficient accuracy in forecasting tropical cyclogenesis and
whether such a forecast could be used as the basis for categorical genesis forecasts in the GTWO.

NHC forecasters subjectively assigned a probability of genesis (0 to 100%, in 10% increments) to each area of disturbed weather
mentioned in the text TWO. The assigned probabilities represented the forecaster's assessment of the chance that TC formation would
occur during the ensuing 48-h period. These experimental probabilistic forecasts remained "in-house" and were not released to the
public in 2007. The probabilistic forecasts were continued in-house in 2008, and these forecasts were used to determine the three-tiered
color-coded categorical forecast that was assigned to each disturbance mention in the GTWO during the 2008 season. The 2007-08
probabilistic genesis forecasts were verified based on TC development from the final NHC best-track data. A more detailed description of
the probabilistic genesis forecast and its verification is presented in Brown et al. (2008)*. The results of the probabilistic forecast
can also be found in the annual verification report issued by the NHC each spring. The report can be found on the NHC website at
www.nhc.noaa.gov/verification/verify3.shtml.

The results of the probabilistic genesis forecast verification suggest that NHC has skill at distinguishing between three broad
categories of genesis likelihood in both the Atlantic and eastern North Pacific basins (Tables 1 and 2). For example, disturbances
assessed as having a low probability of genesis became a tropical cyclone on average less than 30% of the time in both basins, while
disturbances in the high category underwent genesis greater than 60% of the time on average in both basins. These results were used as the
basis for construction of the three-tiered color-coded genesis probability forecast bins that will be used in the GTWO in 2009.

Expected %

Verifying %

Number of Forecasts

<30% (Low)

8

6

916

30-50% (Med)

37

34

243

>60% (High)

70

62

109

Table 1. Binned Forecast Reliability Table for the
Atlantic basin, 2007-08. The expected percentage is an average of all the forecast
probabilities within the bin.

Expected %

Verifying %

Number of Forecasts

<30% (Low)

11

19

540

30-50% (Med)

38

52

166

>60% (High)

69

79

63

Table 2. Binned Forecast Reliability Table for the
eastern North Pacific basin, 2007-08. The expected percentage is an average of all the forecast probabilities
within the bin.